Diode-test the lamp 7408 and 7406 chips

Steps

• Game off, meter on diode test, red lead on pin 7.
• 7408 column chips IC13/IC14: black lead on pins 1-6 and 8-13: expect 0.4 to 0.6.
• 7406 row chips IC12/IC19: test pins 1-2, 5-6 and 8-13 with the black lead: expect 0.5 to 0.7.
• Under 0.2 on any pin means a bad chip. Socketed chips test better out of circuit; a logic probe comparison with a similar chip is better still.

Systems

• Williams System 3
• Williams System 4
• Williams System 6 / 6A
• Williams System 7

Related tips

• Diode-test the lamp column and row transistors
• Diode-test the lamp pre-drivers (2N6427/MPSA14)
• Lamp rows stuck on: the 7406 at IC19
• How the lamp matrix runs 6.3 V bulbs on 18 V
• Tester LED check of the lamp columns
• Tester LED check of the lamp rows
• MOSFET lamp column upgrade
• Light a lamp row by grounding its transistor

Source: Repairing Williams System 3 to System 7 Pinball 1977 to 1984, Part Three, Clay Harrell (pinrepair.com)